Cryogenic refrigeration for crystal x-ray diffraction studies



CRYOGENIC REFRGERATION FOR CRYSTAL X*RAY DIFFRACTION STUDIES Filed Feb. 25, 1967 Ss mmm f. V .V E www i n mw U www l DH M ww mk 5A L n n nited States Patent G CRYUGENIC REFRIGERATION FOR CRYSTAL X-RAY DIFFRACTIUN STUDIES Henry lli. Villaume and Donald S. Collins, Emmaus, Pa.,

assiguors to Air Products and Chemicals, Inc., Allentown, Pa., a corporation of Delaware Filed Feb. 23, 1967, Ser. No. 617,905

5 Claims. (Cl. 62-514) ABSTRACT F THE DISCLOSURE An apparatus for the cryogenic refrigeration of a specimen to be studied by X-ray diffraction. The specimen is mounted within a vacuum jacket and cooled with a miniature, open-cycle, Joule-Thomson refrigerator.

This invention relates to a cryogenic refrigeration system for single crystal X-ray diffraction studies and more particularly, to such a system having a miniature cryogenic refrigeration unit for cooling the crystal which is to be studied under X-ray beams with a diffraction device such as a goniometer.

Goniometers are mechanisms or instruments for measuring angles or for setting objects in known orientations. For example, a goniometer may be employed to rotate and adjust a single crystal in the path of the incident X-rays. Thus, using a goniometer it is possible to completely study a tiny crystal, roughly about the size of a grain of salt, by an incident X-ray beam. A Weissenberg photogoniometer is also suitable for such studies. The vWeissenberg photogoniometer has cylindrical film shielded and axially advanced in such a way during exposure that the spots of a single layer line are recorded consecutively along the direction of travel of the film.

For such studies, it is essential or at least highly desirable to maintain the crystal specimen at a very low and uniform temperature in order to slow down molecular activity and obtain a relatively constant space lattice. Very careful refrigeration is also required when crystals of materials which are normally in the gaseous or liquid state at room temperature are prepared for study. ln addition, low temperatures are essential for crystals which effioresce or evaporate, i.e., crystals of materials such as mothballs, Dry Ice, hydrated compounds, etc.

The refrigeration problem for diffraction device crystal studies has never been solved satisfactorily. In the past, cooling of the crystal sample has been effected by passing a stream of cold gas over the sample from a fixed source of cryogen, for example, liquid nitrogen, requiring the control of an external refrigerant present in large fixed supply. This technique has the obvious disadvantage of temperature fluctuation and the inability to completely lower the temperature of the crystal to the temperature of the liquid being employed as coolant. Furthermore, prevention of crystal frosting is difficult to accomplish.

In another approach, the sample holder for the crystal sample and the sample itself have been immersed in a small Dewar containing liquid nitrogen. While the sample is effectively cooled by direct heat contact with liquid nitrogen, this system is particularly limited with respect to the degree of arc movement possible without spilling liquid nitrogen from the Dewar. Accordingly, in this case the diffraction device base, X-ray source and detector must be moved about the sample after fine adjustment in the location of the crystal sample to a common center.

In yet another system, the entire diffraction device is placed in a cooling chamber. It is apparent that size and cost of such a chamber, the reduction in movement possible for the sample or X-ray source and the difficulties with construction materials at very low temperatures militate against such a system.

It has now been found that direct conductive cooling of crystals which are to be studied under X-ray beams may be obtained employing a miniature, open-cycle, Joule- Thomson refrigerator, or a cryostat, which is mounted directly to a device, such as a goniometer, for setting a crystal in known orientation. In this system, the sample is maintained at a substantially constant temperature in direct heat conductive contact with cryogenic liquids. By mounting the cryostat directly onto an adjustment eucentric goniometer which, in turn, is mounted on a full circle or partial circle Eulerian cradle, a Weissenberg camera or other X-ray diffraction device, a single crystal may be completely studied by an incident X-ray beam. The small size and design arrangement of the cryostat permits its installation in such a manner that none of the adjustment motions (provided by the euceritric goniometer) or the programmed motions provided by the Eulerian cradle or other X-ray diffraction device (to permit full exploration of the crystal facets by the X-ray beam) are impeded.

The present invention utilizes liquefiable cylinder gas under pressure to produce refrigeration in situ within the miniature cryostat and accordingly, does not require insulation of the gas supply line to the cryostat` Other advantages of the present invention include the elimination of frost-up, i.e., a commonly prevailing problem caused by the formation of ice on the cold crystal, resulting in blurring, obscuring or fuzzing of the X-ray patterns. This latter problem is overcome by providing a vacuum shroud or jacket, of suitable material such as aluminum, brass, stainless steel, etc., around the cryostat and having a thin sheet of beryllium or other suitable material as a window in the vacuum jacket which permits X-ray passage with a minimum of interference.

The Joule-Thomson cycle per se does not form part of the invention. Refrigeration systems operating on the Joule-Thomson cooling effect principle are well known as seen in U.S. Letters Patent No. 3,188,824 and 3,205,679. Such Joule-Thomson refrigeration systems or cryostats utilize the cooling phenomena attendant a rapid expansion of a non-ideal gas without doing work. Before such cooling will occur, the refrigerant gas must be at a temperature below its inversion temperature.

In a preferred embodiment of the invention, though in no way limited thereto, the refrigeration system comprises a cryostat in which a coil of high heat transfer or extended surface tubing, for example, finned tubing, is wound about a mandrel and is inserted snugly within a sleeve, the coil-wound mandrel and the sleeve together providing a counter-current flow heat exchanger. An expansion zone or chamber is provided near the end of the system for expansion and possible condensation of refrigerant. Vapors from the expansion zone continuously flow from this chamber to a vent and Contact the coiled tubing along its extended surface while passing through the annular space provided between the mandrel and the sleeve, thereby pre-cooling the incoming refrigerant.

After passing through the coil, the precooled refrigerant is conducted to the expansion zone and discharged therein through an orifice at a rate of flow sufficient to effect the desired cooling.

The invention will be further clarified by the following description and from the accompanying drawings, wherein:

FIGURE 1 is a schematic illustration of a refrigeration unit mounted within a goniometer and supplied with pressurized gas; and

FIGURE 2 is an enlarged schematic cross-sectional view of a miniature refrigeration unit, its alignment system and vacuum jacket.

In FlGURE l, appropriate cylinder gas, such as air, Freon, nitrogen, argon, hydrogen, helium, etc., under pressure of up to about 2,500 p.s.i. (the pressure being dependent on the refrigerant collected) is passed from tank through line 11. If required, water vapor is removed frorn the pressurized gas in adsorber 12 which contains a suitable adsorbent, such as molecular sieve material, activated carbon, etc. The pressure of the gas leaving the adsorber is monitored by gauge 14 and then regulated by a high-pressure regulator 16 to reduce the gas pressure from cylinder pressure to a regulated lower pressure. Typically, for air or nitrogen the pressure is reduced to between about 1500 p.s.i. and a minimum of about 300 to 400 p.s.i.

The gas then flows through a throttle valve 18 by which very precise pressure control is obtained and thereby precise control of temperature in cryostat 23. A highpressure drop is thus available for quick cryostat cooldown and the gas rate may be adjusted to maintain a desired cryostat temperature following Cooldown. A pressure gauge 20 may be present in the gas flow circuit following throttle valve 18 to measure gas pressure to the cryostat and as an aid in establishing temperature in the cryostat.

Finally, the gas is passed through conduit line 21 into cryostat 23 which is mounted in goniometer 24. Normally, three separate and distinct lines are attached to the cryostat, viz., (l) high-pressure gas line 21; (2) a thermocouple line (not shown); ad (3) a vacuum line (not shown). The thermocouple line can be connected to a pyrometer to measure temperatures in the cryostat and provision can also be made for a potentiometer outside the cryostat for fine reading of the ther-mocouple EMF. The cryostat is maintained in a vacuum jacket to minimize heat leak and avoid frost-up of the crystal specimen. Vapors from refrigerant employed in the cryostat are normally vented, but can be recycled for further utilization.

The cryostat, its alignment system and vacuum jacket are shown in more detail in FIGURE 2. In this figure, the cryostat assembly is indicated generally at 23. Vacuum jacket or shroud 25 has a gastight and vacuum stable window area 26 which is pervious to X-rays. This window area may be made of any material which does not interfere substantially with the passage of X-rays, e.g., beryllium, acrylic plastic such as Lucite or Plexiglas, amorphous plastic films such as Mylar, etc. Using a combination of vacuum jacket window materials, such as beryllium and Plexiglas, a photographic background may be readily changed.

Specimen holder 28 is a removable metal plug friction fitted Within a metal closure member or plug 29 which terminates the cryostat. The sample 32 is positioned on a mounting pin 27 in specimen holder 28. Thus, cooling effect is transferred to sample 32 from condensed refrigerant 34 by conduction through elements 27, 28 and 29. This mounting arrangement permits rapid interchange of samples and quick cooling by solid-solid conduction. Because of its conductivity, copper is a particularly preferred metal for the removable metal plug.

Crystal placement is normally in the exact center of the several arc circles of a goniometer. For precise adjustment, a Z axis or longitudinal axis adjustment nut 36 combined with bellows 37 permits alignment of the crystal specimen within the vacuum jacket. Specifically, bellows 37 are attached to a cylindrical sleeve carrier member 39 which is in threaded engagement with locking element 40' and in fixed attachment with sleeve 42. Bellows 37 are also attached to a cylindrical casing member 43 which is in threaded engagement with longitudinal axis adjustment nut 36 and in xed attachment with vacuum jacket 25. A vacuum seal is maintained between cylindrical casing member 43 and vacuum jacket 25 by means of a threaded engagement 45 and O-ring seal 46. Adjustment of the posi- 'tion of the specimen or sample 32 along the Z axis of the cryostat assembly is thus accomplished by turning longitudinal axis adjustment nut 36 to effect a desired raising or lowering of the cylindrical sleeve carrier member 59 and hence the entire cryostat inside vacuum jacket 25.

Refrigerant gas is introduced into the cryostat assembly by means of high-pressure gas conduit line 21. This gas passes through a heat exchanger comprising a core with coils or tubes 48 wound over a mandrel 49 and bounded by sleeve 42. The gas is emitted from tubes 48 through an orifice 51. Vapors from refrigerant 34 pass in indirect heat exchange relationship with the incoming refrigerant through the annular space containing heat exchange tubes 48. These vapors are either vented or recycled (by means not shown) for further utilization.

A vacuum is maintained in the cryostat assembly around sleeve 42 by means of vacuum conduit line 53. Thus, sleeve 42 and Vacuum jacket 25 provide double Wall insulation and the space between these double walls is maintained under vacuum.

Temperature measurements inside the assembly are obtained by employing thermocouple lines 55 which pass through conduit 56 to thermocouple sensor 57.

The cryostat assembly is attached to goniometer 24 by means of an O-ring seal 59 and a threaded engagement 60.

In a commercial design, a standard gas cylinder (220 s.c.f. bottle at 2200 p.s.i.) of commercial purity nitrogen was operated for ten hours to obtain a temperature range of to 300 K. and a net refrigeration capacity of about one watt at 80 K. Cooldown time for the cryostat which was mounted in an adjustable eucentric goniometer which, in turn, was mounted in afull circle Eulerian cradle, was 5 to 8 minutes. The vacuum jacket for the cryostat was made of stainless steel and had a beryllium window of approximately 0.01 inch thickness. The system could be operated throughout the range of adjustment for the goniometer.

Even lower temperatures may be obtained by employing a cascade Joule-Thomson refrigerator. Thus, by introducing nitrogen and neon through separate coils wound around the mandrel the nitrogen may be used to precool the neon and thereby obtain a temperature as low as 30 K. Similarly, using nitrogen to precool hydrogen a temperature as low as 20 K. may be Obtained. A temperature as loW as 4 K. may be obtained using a cascade Joule- Thomson refrigerator in which nitrogen is employed to precool neon or hydrogen and the neon or hydrogen is in turn employed to precool helium.

Although emphasis has been placed on the mounting of the cryostat on an adjustment eucentric goniometer, it is to be understood that cryostats capable of being mounted on stages providing adjustment only on the X and Y axes are also contemplated. In such an embodiment, specimen location on X, Y, and Z axes can be accomplished, but there are no arc adjustments provided.

Thus, the present invention provides and maintains constant conductive cooling at very low temperatures which is essential for X-ray diffraction analyses of a broad range of crystalline organic and inorganic compounds. In the above commercial design, cryogenic liquid is less than onefourth of an inch from the specimen crystal with a highly conductive direct heat transfer path between the liquid and crystal. Refrigeration is obtained using a standard compressed gas cylinder of liqueable gas. Due to the size of the miniature cryogenic refrigerator or cryostat (approximately three inches and weighing less than 61/2 oz.), the total unit as mounted fits within any goniometer (normally, a full circle goniometer is as small as six inches in diameter) or any Weissenberg or precession camera provided with a support for an eucentric goniometer. There is no loss of phi, chi or omega motion since the mounting of the cryostat is directly on the goniometer. Particularly advantageous is the Z or longitudinal axis adjustment means which allows movement of the sample within the vacuum jacket. Finally, frost-up on the cold crystal, which causes a blurring of diffraction patterns, is eliminated using the vacuum jacket.

Obviously, many other modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

1, A cryogenic refrigeration system for X-ray diffraction studies of crystals which comprises: an elongated, Joule-Thomson cryogenic refrigerator, said refrigerator being so miniaturized such as to be mounted wholly within a goniometer, said refrigerator including an elongated heat exchanger coil having a warm end and a cold end, said warm end being connected to a source of high pressure gas, said cold end terminating in a Joule-Thomson expansion orice for producing a cryogenic refrigerant liquid 'from said gas, means for mounting a crystal sample on the cold end of said refrigerator in heat exchange relationship with said cryogenic liquid `for cryogenically cooling said sample thereby, a `vacuum shroud spaced from and surrounding said refrigerator, a partial vacuum between said shroud and said refrigerator, said shroud including a Window pervious to X-rays, and means connected between the warm end of the refrigerator and said vacuum shroud for longitudinally adjusting the position of said sample mounted on said refrigerator relative to said X-ray-pervious window while maintaining said partial vacuum therebetween.

2. The refrigerator system as claimed in claim 1 Wherein said heat exchanger coil is helically wound about a hollow mandrel, a plug element closing the cold end of said hollow mandrel, said plug being within and adjacent the cold end of said sleeve, a temperature sensor disposed within said plug element and having leads extending through said hollow mandrel to the warm end thereof, said plug element being immediately adjacent said Joule-Thomson orice whereby said sensor detects the cryogenic temperature of the cryogenic liquid refrigerant in heat exchange relationship with said sample.

3. The refrigerator system as claimed in claim 1 wherein said refrigerator includes a sleeve surrounding said coil and carrying the sample mounting means, and an eX- pandable and contractable 'bellows element having one end connected to said sleeve and the other end connected to said lvacuum shroud whereby said sleeve maintains said partial vacuum therebetween during adjustable movement of said sleeve relative to said shroud.

4. The refrigerator system as claimed in claim 3 including a rotatable adjustment element having a rst portion connected to said sleeve and having a second portion threadedly connected to said vacuum shroud whereby manual rotation of said adjustment element longitudinally adjusts the position of said sample relative to said X-ray window.

5. The refrigeration system as claimed in claim 4 wherein the connection between said rst portion of said adjustment element and ysaid sleeve includes a pair of connector elements, said first portion of said adjustment element being loosely clamped between said connector elements whereby said adjustment element rotates relative to said sleeve and produces longitudinal movement thereof relative to said shroud.

References Cited UNITED STATES PATENTS 2,909,908 10/ 1959 Postuhov et al. 62-514 3,289,424 12/1966 Shepherd 62-55 3,292,383 12/1966 Charles et al. 62/-55 3,306,075 2/1967 Cowans 62-514 3,327,491 6/1967 Andonian 62-514 LLOYD L. KING, Primary Examiner. 

